1. Field of the Invention
The present invention relates to a golf shaft, a forming method therefor and a golf club.
2. Description of the Related Art
Generally speaking, a golf shaft is made by the filament winding method in which filaments as resin-impregnated high-strength high-elasticity fiber are wound on a mandrel and baked, or by the sheet winding method in which a prepreg as a sheet composed of such filaments aligned is wound on a mandrel and baked, so it has some degree of flexibility and a high rigidity. Also, when carbon fiber is used as high-strength, high-elasticity fiber, a lightweight golf shaft with a high strength can be produced. A golf club which uses this type of golf shaft is used by many users.
Regarding a golf shaft using carbon fiber, the flexural rigidity (bending strength), torsion strength, crushing strength and so on of a product (golf shaft) are considerably affected by the elasticity of the fiber used, the fiber's orientation with respect to the mandrel core (axis), its position along the length of the shaft or in the diametric direction and other factors.
When a head and a grip are attached to a golf shaft to make a finished golf club, the golf shaft strength characteristics as mentioned above also largely influence the carry and ball direction. For this reason, efforts to develop a golf shaft which meets user needs have been actively conducted.
Recently, in order to obtain a characteristic which cannot be achieved by carbon fiber only, many golf shafts which use a combination of carbon fiber and other materials have been developed: one example is that metal wires made of boron, titanium or a similar metal are arranged along the length of the shaft for the purpose of improving propagation of vibration upon impact and another is that metal wires are disposed as a reinforcement to a head mounting area with a smaller diameter and a low bending strength or torsion strength.
On the other hand, as a recent trend, there has been growing demand for the use in a golf shaft of a special metallic color which produces an aesthetic design effect, so golf shafts whose surface is coated with a metallic paint or metal-evaporated golf shafts are increasingly used.
However, although a metallic color can be developed on the golf shaft surface by decoration techniques such as coating with metallic paints and evaporation, it does not give the shaft an appearance with a sense of depth or a three-dimensional touch.
A metallic color can also be developed by densely aligning wires of titanium or other metal as mentioned above in a manner that they appear on the surface of a golf shaft. However, in that case, the weight is unavoidably increased and ideal strength characteristics are difficult to achieve.
In a golf shaft, a layer closer to the outer layer more affects the bending strength. Usually metal wires have circular cross sections and, therefore, occupy a large proportion of the shaft thickness, so it is difficult to achieve ideal strength characteristics (particularly bending strength) in terms of a strength-to-weight ratio.
Furthermore, in order to ensure visibility, the recent tendency is that metal wires are located in a layer nearer to the shaft surface than in an inner layer where metal wires were located previously. This means that many more metal wires have to be used than when metal wires were used in an inner layer. Besides, the weight of these metal wires is larger than carbon fiber. The result is an increase in the overall weight of the golf shaft.
In addition, because metal wires have a poor adhesion and are easy to peel off, a larger volume of synthetic resin for bonding them must be used to prevent their peeling, leading to an increase in the overall weight of a golf shaft.